The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a user equipment (UE) 150 is wirelessly connected to a base station (BS) 110a commonly referred to as an evolved NodeB (eNodeB), as illustrated in FIG. 1a. Each eNodeB 110a-c transmits signals associated with a cell 120a-c, and are connected to the core network. In LTE, the eNodeBs 110a-c are connected to a Mobility Management Entity (MME) 130 in the core network.
UE Positioning
UE positioning is a process of determining UE coordinates in space. Once the coordinates are available, they may be mapped to a certain place or location. The mapping function and delivery of the location information on request are parts of a location service which is required for basic emergency services. Services that further exploit location knowledge or that are based on the location knowledge to offer customers some added value are referred to as location aware and location based services. The possibility of identifying a wireless device's geographical location in the network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, and emergency calls. Different services may have different positioning accuracy requirements imposed by an application. Furthermore, requirements on the positioning accuracy for basic emergency services defined by regulatory bodies exist in some countries. An example of such a regulatory body is the Federal Communications Commission regulating the area of telecommunications in the United States.
The major driving force for location based services is emergency positioning, denoted E-911 positioning in North America. The accuracy requirements for E-911 positioning are quite stringent, which has lead to a technical solution with Assisted Global Positioning System (A-GPS) as the main positioning method. One or several fallback positioning methods are also normally implemented to cover up where A-GPS performs badly, e.g. indoors or in urban canyons. Common fallback positioning methods are cell identity (CID) positioning, timing advance (TA) positioning, fingerprinting positioning as well as Time Difference Of Arrival (TDOA) based positioning methods in the uplink or downlink.
A TDOA method relies on measurements on known reference radio signals from multiple BSs. The measurements are performed by means of correlation with the known signals from the BSs measured upon. One example situation in an LTE network is depicted in FIG. 2.
Assuming that the measurements are successful for a number of cells, three of which are depicted in FIG. 2, the following relations between the measured Time Of Arrivals (TOA) in the terminal or UE, the transmission times from the BSs, in this case the eNodeBs, and the distances between the UEs and the eNodeBs apply:
                    t                  TOA          ,          1                    +              b        clock              =                  T        1            +                                                            r              1                        -                          r              Terminal                                                /        c                                ⁢    ⋮                      t                  TOA          ,          n                    +              b        clock              =                  T        n            +                                                            r              n                        -                          r              Terminal                                                /                  c          .                    
Here tTOA,i, i=1, . . . , n denotes the measured TOAs in the UE, Ti, i=1, . . . , n denotes the transmission times from the eNodeBs and c is the speed of light. The boldface quantities are the vector locations of the eNodeBs and the UE. The variable bclock denotes the unknown clock bias of the UE with respect to the cellular system time. In TDOA positioning, the time of arrival differences with respect to the serving site are formed according to:
            t              TDOA        ,        2              =                            t                      TOA            ,            2                          -                  t                      TOA            ,            1                              =                        T          2                -                  T          1                +                                                                        r                2                            -                              r                Terminal                                                          /          c                -                                                                        r                1                            -                              r                Terminal                                                          /          c                                        ⁢    ⋮              t              TDOA        ,        n              =                            t                      TOA            ,            n                          -                  t                      TOA            ,            1                              =                        T          n                -                  T          1                +                                                                        r                n                            -                              r                Terminal                                                          /          c                -                                                                        r                1                            -                              r                Terminal                                                          /                      c            .                              
In these n−1 equations, the left hand sides are known, including some additional measurement error, provided that the time of transmission differences, also denoted the real time differences, may be measured. Furthermore, the locations of the Bss ri, i=1, . . . , n, may be surveyed to within a few meters and are thus known as well. What remains unknown is the UE location, i.e.:rTerminal=(xTerminalyTerminalzTerminal)T.
In a more common case a two dimensional positioning is performed instead:rTerminal=(xTerminalyTerminal)T.
It then follows that at least three time of arrival differences are needed in order to find a UE position in three dimensions (3D) and that at least two time of arrival differences are needed in order to find a UE position in two dimensions (2D). This, in turn, means that at least four sites need to be detected for 3D UE positioning and at least three sites need to be detected for 2D UE positioning. In FIG. 2, three sites 210a-c are detected by the UE 250, and the UE position estimate is found as an intersection 230 of two hyperbolas 240a-b corresponding to the measured time differences. In practice, accuracy can be improved if more measurements are collected and a maximum likelihood solution is introduced. There may also be multiple false solutions in cases where only a minimum number of sites are detected; In FIG. 2 the two hyperbolas 240a and 240b actually have two intersections and the positioning does not give one correct solution. Another example is when there are three hyperbolas which do not intersect in the same point. If false solutions exist or not thus depends on the number of hyperbolas and on the geometry of the BSs involved. Observed TDOA (OTDOA) is the TDOA positioning method that has been standardized for LTE in 3GPP Rel-9.
A positioning node is a physical or logical entity that manages positioning for a so called target device, e.g. a UE, and is in a control plane architecture referred to as an Evolved Serving Mobile Location Center (E-SMLC). As illustrated in FIG. 1a, the E-SMLC 140 may be a separate network node connected to the MME 130, but it may also be a functionality integrated in some other network node. An overview of the positioning architecture in LTE is shown in FIG. 1b. As already mentioned, the E-SMLC is the control plane network node for positioning. The corresponding user plane node is the Secure User Plane Location (SUPL) Location Platform (SLP). Both the E-SMLC and SLP node can communicate with the UE over the LTE Positioning Protocol (LPP). This communication is transparent to the serving eNodeB. The LPPa protocol may be used by the E-SMLC to derive the eNodeB configurations or eNodeB based positioning solution. When receiving a OTDOA positioning request, the E-SMLC may e.g. request positioning related parameters from eNodeB via LPPa. The E-SMLC then assembles and sends assistance data and the request for the positioning to the target wireless device, e.g. the UE, via LPP. The assistance data is provided to the UE in a transparent manner for the serving eNodeB.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning, so called positioning reference signals (PRS) have been introduced. To improve hearability of the PRS, i.e., to enable detecting the PRS from multiple sites and with a reasonable quality, positioning sub frames have been designed as low-interference sub frames. It has thus also been agreed that no data transmissions are allowed in general in positioning sub frames.
To limit the system impact of using OTDOA in the network, the PRS may be configured to be transmitted very seldom in time. The current possible configurations are a set of sub frames with PRS, referred to as a positioning occasion, every 160 ms, 320 ms, 640 ms and 1280 ms. The set of sub frames with PRS may be 1, 2, 4 or 6 consecutive downlink sub frames. FIG. 3 illustrates positioning occasions of N_prs=6 consecutive sub frames, occurring every T_prs=160 ms.
As PRS from multiple distinct locations need to be measured for OTDOA positioning, the UE receiver may have to deal with received PRS that are much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE would need to do signal search within a large window which would impact the time and accuracy of the measurements as well as the UE complexity. To facilitate UE measurements, the network transmits assistance data to the UE, which includes among others, reference cell information, neighbor cell list containing physical cell identities of neighbor cells, the number of consecutive downlink sub frames (N_prs), PRS transmission bandwidth and frequency.
The UE measures the observed time difference between the cells provided in the assistance data compared to a reference cell. The reference cell is also part of the assistance data. With OTDOA, the UE measures Reference Signal Time Difference (RSTD) which is a relative timing difference between a neighbor cell and a reference cell. In LTE Rel-9 the UE feedbacks these observed time difference measurements to the E-SMLC or SUPL node, which calculates the positioning of the UE. From a theoretical perspective it would also be possible to let the UE calculate its own positioning and feedback the position back to the E-SMLC and the SUPL node.
Energy Saving Mode for BS
The LTE Rel-8 and subsequent releases are standardized for many different transmission bandwidths. In Rel-8 and Rel-9 the defined transmission bandwidths span from 1.4 MHz to 20 MHz. While a large transmission bandwidth is needed during times of high load, the large transmission bandwidth does not provide benefits during low load situations. On the contrary, a large transmission bandwidth implies higher energy consumption since Cell specific Reference Signals (CRS) need to be transmitted over the full system bandwidth at all time. It is therefore desirable to tune the transmission bandwidth depending on the available load.
The transmission bandwidth of a cell is contained in the system information transmitted on the Physical Broadcast Channel (PBCH). However, there is no procedure defined for how a UE shall behave in case of a transmission bandwidth change for a cell. Therefore, it is in practice not possible to change the bandwidth of a cell. Instead, to reduce the bandwidth of a cell served by a certain BS, a new narrow-band cell that is transmitted from the same physical BS may be added, and the current wide-band cell may be removed.
Adding a new cell is a procedure that requires no additional hardware in the BS. Thus, even though it is not possible to change the bandwidth of an LTE cell, it is possible to add and remove cells of different bandwidths in order to adapt e.g. the bandwidth to the current capacity requirement, thus making it possible to save energy during periods of low capacity need.
However, such energy saving methods implies that an eNodeB will transmit its signals with different transmission bandwidths depending on if it is in an energy saving mode or not. This may then affect all UEs in neighboring cells that are performing OTDOA measurements including a measurement of PRS from this eNodeB. The UEs and the eNodeB may in this case have a different understanding of the transmitted bandwidth. The UEs thus need to be informed about that the measured eNodeB has changed its transmission bandwidth, in order for them to be able to update the measured bandwidth for positioning. The UEs may only get this information if the SLP and E-SMLC are aware of the eNodeB's changes between the energy saving mode and the high capacity mode. Such a solution would be very complicated as the eNB needs to continuously update E-SMLC and SLP node about it energy saving status. Furthermore, the SLP node may not be directly under radio access network control, which may set further limits on what is possible. Another problem is that the UE OTDOA measurement performance is connected to the PRS transmission bandwidth. The larger the transmission bandwidth for PRS is, the better is the performance. Therefore, if the eNodeB would lower its transmission bandwidth, it would also lower the OTDOA performance. This is not desirable even if the network for the time being is low loaded, since the OTDOA measurements may be related to the positioning of a UE for a call to E-911, and for such situations an exact positioning is critical.